We’ve got clients who are interested in a big cistern to store rainwater, and any solar heating ideas we can come up with. Does anyone have experience with using solar heated hot water to heat the water in a big tank, say 1500 gallons, then extracting that heat for domestic use?
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I lived in the rural parts of Santa Clause county( Bay area) for ten years just outside Gilroy, the garlic capital of USA. We maintained household water supply from a well coupled with storage in a 5000 gallon cpvc water tank, black in color, which sat out in the direct sun. The water never got very hot. The interior of the tank, when the level was down slightly, was unbearably hot on the inside but the water inside the tank was quite cool, even after sitting all afternoon.
Water requires a large amount of heat to raise it's temperature (one calorie/gram/degree C in fact) and whatever heat is gained during the day in direct sunlight is lost when it sits there all night long. If you go with panels on the roof and active pumping/recirculation then you'll need to insulate that tank if you wish to retain the heat for any length of time. Best bet would be to recommend they install an insulated secondary tank just for solar heated water, in the range of 100 - 150 gallons or so. Make sure the system has pressure relief valves, auto shut off in case of water loss, drain system for winter maintenance etc. and it should work just fine. If they're thinking of just letting the sun warm the tank directly in a really passive fashion, then it's not going to work. I would not recommend using collected rain water for drinking unless you install some sort of filtering and bug killing system (uv light system)inline before the tap. It's a good idea to do that if your on a well anyway. Make sure that you don't contaminate the water as it's collected either, don't collect water from gutters lined with lead or that use lead at the seams etc.
From a different perspective, people have been drinking collected rain water in cisterns for generations so it can't be all that bad. Don't know how long they lived on it but they did live for at least a few years anyway.
I'm not sure how many sunny days a year we get here in Maine but I'm pretty sure it's not enough to consider a totally passive cistern heating system. Passive solar would be considerations to window placement, roof overhang etc. The cistern would be on some sort of active system. Good points on contamination, insulation, and maintenance.
not personal experience, but I read about this months ago...
http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/solarshed.htm
I hope this helps.
i've read that artical 10x... and have a plan to bluild a like unit... except on a large flat roof...
great artical
p
I have designs on it as well, pun intended.
My first issue of Mother Earth News was Jan. 08--just missed that article! I'll save that one.
There is a small but real risk of legionnaires' disease in hot water systems where there is sufficient material in the water to nourish the bacteria. Legionnella will grow with temperatures between about 70 degrees and 120 degrees F. Legionnella reportedly is not easily contracted, but the death rate of those getting the bacteria has been cited at over 10 percent. The fatality rate is, of course, highest in those with immature or compromised immune systems.
If the water is pristine, there is probably a very small risk of contracting the disease. I am interested in solar hot water heating systems, but I am not sure just how much risk I want to take or even just how to fully quantify it. I may want to make sure that my system will maintain a temperature above 120 degree F. for significant periods or perhaps figure a way to cycle the water to 140 degrees F. for whatever the appropriate periods might be to minimize the bugs.
A pretty good summary of legionnaires' disease is at:
http://www.iphe.org.uk/health/legionnaires.html
It is a closed loop then it does not make any difference..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
We live in the islands and every house here has a cistern, with rain collection. I have many stories on what ends up in the tanks. (Iguana in one)
Everything that lands on the roof affects the cistern.....bird poop, frogs, insects etc. they somehow get past the best screening methods
We have a good triple stage filter and dedicated cooking and drinking faucets, other uses, showers toilets etc have simple sediment filters.
Most cisterns here are concrete 12,000 gals plus or large fibreglass tanks 1000gals to 8000 gals, so they take up large spaces in the building.On the solar end it seems if you have enough sun you could easily heat a 1500 tank. Depending on the sun (insolation) three to four 4x8 active panels will heat alot of water, You will need to insulate the tank and treat it like a large domestic HW system.
You could use a closed loop with a heat exchanger to heat potable water for home use.
I can't help but think that a system with a large capacity for storage and heated by solar would require some maintenence, just like a swimming pool, you would have to test, treat, and maybe even keep records.
There is no such thing as a free lunch.
Good point on Legionnaire's disease.
Since the water in the tank probably wouldn't get super hot in the winter, I wonder if it might work well with a radiant floor heating system? It would be a closed loop. You could have one coil running through the tank with hot water from the solar panels, and another loop as a pre-heater for otherwise conventionally heated domestic hot water.
Does anyone have experience
Not yet. I've got 480 sq ft of flat plat collector for the next house which will feed a 650 gal tank. As space heating isn't an issue (PAHS) it's primarily for the indoor pool and domestic hot water. My (steel) tank sits in a utility room.
Drain-back works well in that article. I'll use antifreeze and place the panels on the roof. With the panels, I salvaged 4 liquid/liquid heat exchangers, enough pumps and controls to last a lifetime. Around here, it's been easy to find defunct systems to scavenge.
If you're thinking 1500 gal I'd be looking at ferro-cement. Donnie Shatzberg wrote a nice self-published book "Build Your Own ferro-Cement Water Tank" with designs for up to 8000 gal. That's 6' tall, 15' d.
My ferro-cement experimenting begins in a few weeks, not a water tank. If it doesn't start raining, I'll be considering rainwater harvesting. Last substantial rain was early July.
If you want to pre-determine effectiveness, Other Homes and Garbage ISBN 0-87156-141-7 will take you painlessly through the calculations. http://www.gaisma.com/en/location/portland-maine.html gives insolation and % of possible sunshine for Portland. I notice January is listed (Clearness) at 49%, where my (old) OHaG chart says 55%.
Have fun.
PAHS works. Bury it.
Thanks Tom--
One of these days I'm going to drive down there and talk you into giving me a tour--
LOL... you'd be the first BTer. Not that I haven't offered. Even door prizes. Need a wood-fired boiler that'll take 4' logs? Or a 500 bf piece of oak? SG didn't make it. Something to do with truck size. Sphere keeps threatening.
Ferro-cement is scheduled for Sept. sometime. I think m2akita's finally coming out. I offered to trade him my no-carp-talent help in building a horse run-in shed. My need is doubling my rooftop raised veggie beds, replacing my rotted oak timbers. Getting more serious about food production.
BTW, my panels will provide 130% of my pool and domestic hot water needs. So I'm installing a few cast radiators, found some really nice ones to recycle. Hope your clients get on with it.
PAHS works. Bury it.